The present invention relates generally to increasing the performance of a motor. More specifically, the present invention relates to the increasing of the horsepower and efficiency of an induction motor by applying a motor input voltage and frequency greater than the motor's rated voltage and frequency.
In the past, induction motors, both single phase and multi-phase, were designed to operate from standard line (main) voltages and frequencies that were available from the power distribution system of the facility where the motor was being operated. The use of line voltages and frequencies typically limited the output of a motor to one speed and one horsepower because the motor's output speed is related to the input frequency to the motor and the motor's output power is related to the input voltage to the motor.
Next, variable speed drives (VSDs) were developed that could vary the frequency and/or voltage that was provided to a motor. This capability to vary the input frequency and voltage to a motor resulted in a motor that was capable of providing a variable output speed and power to the load of the motor. Furthermore, for the motor to provide a relatively constant output torque to the load, regardless of the speed of the motor, the motor had to be operated in a constant flux or constant volts/Hz mode. The operation of a motor in a constant flux or constant volts/Hz mode requires that any increase or decrease of the output frequency of the VSD that drives the motor be matched by a corresponding increase or decrease of the output voltage of the VSD, and vice versa. In addition, when a motor is operated in the constant flux or constant volts/Hz mode, the current drawn by the motor will remain relatively constant, especially in situations where the motor is being used with a constant torque load, such as a positive displacement compressor used in air conditioning equipment.
One problem with trying to operate a motor in the constant flux or volts/Hz mode using a variable speed drive is that most variable speed drives are not capable of providing an output voltage to the motor that is any greater than the input AC line voltage provided to the variable speed drive. This limitation on the output voltage of the variable speed drive results in the variable speed drive not being able to provide a constant torque to the load when the speed of the motor is increased (as a result of an increase in the output frequency of the variable speed drive above the line frequency provided to the variable speed drive) because the output voltage of the variable speed drive cannot match the increase in the output frequency.
Furthermore, the selection of a motor for use with a particular load is dependent upon a variety factors including the input voltage to the motor. A motor is usually selected to have a rated voltage that is equal to a standard line voltage or the variable speed drive's rated output voltage, which is equal to its input voltage, if applicable.
Therefore what is needed is a system and technique for operating a motor to obtain a larger output horsepower by using higher input voltages and frequencies without having to use a motor with a higher voltage rating.